The machine-learning engine Lc0 (nicknamed "Leela") is in undefeated clear fourth place behind the world's top three traditional chess engines—Houdini, Komodo, and Stockfish—halfway through the Computer Chess Championship double-round-robin.

The inaugural Chess.com Computer Chess Championship event is called CCCC 1: Rapid Rumble. In stage one, each engine plays every other with both White and Black to determine the top eight to advance to the next stage.

After 276 games, all 24 of the engines have played one another once, and will reverse colors to wrap up the round-robin. Houdini sits in clear first place after the first half with 19/23; Komodo and Stockfish share second and third at 18.5 points.

While the success of Houdini, Komodo, and Stockfish was largely predictable as those engines dominate the computer-chess ranks, the strong push (and hype!) by Lc0 at 16.5 points is the big story of the tournament as fans flock to watch the self-taught engine's games day and night.

Lc0, which learns chess by repeatedly playing games against itself and using neural networks to organize the lessons, has wowed computer-chess experts and casual fans alike with its intuitive, creative, and almost-human approach to the game. The Lc0 project is developed by its community and was inspired by AlphaZero, a private machine-learning chess demonstration by Google's DeepMind.

None of the engines in the top four lost a single game in the tournament's first half. While that result was somewhat expected for the top three engines, the inclusion of the previously error-prone Leela in the undefeated group comes as a welcome surprise for advocates of an artificial-intelligence approach to chess mastery.

The CCCC 1 crosstable after 276 games. Click the image for a larger version.

Lc0 benefited from some early adjudications per the CCCC official rules, but has played stellar chess throughout the first half of the stage. Due to the adjudications, its point total is one point higher than the sum of all results in the crosstable.

Its win against the engine Nirvana is a candidate for the game of the tournament, prompting IM Daniel Rensch to analyze the masterpiece.

Rensch also recorded a video lesson about the game, available below:

Tournament statistics:

At the halfway point of the round-robin, 44.9 percent of the stage's 276 games have been drawn. White won 33.6 percent, and Black won 21.3 percent.

Lc0 itself played more peaceful chess than the overall group, drawing 15 of its first 23 games for a 65.2 percent clip. Significantly, Leela's wins have been spread in the crosstable's bottom two-thirds—after drawing with the top seven engines, Lc0 has won and drawn with the middle and bottom engines at roughly the same rate.

This is in stark contrast to Houdini, Komodo, and Stockfish, which all beat up on the bottom engines, with just two draws and 16 wins in their 18 games vs the bottom-six engines.

Despite the lack of opening book, and each engine (aside from Leela and its stored neural network) playing the starting position completely from scratch, the engines chose just three moves to open 98 percent of the tournament's games.

According to analysis at the LC0 blog, 1. e4 was the most popular opening move, played 55.0 percent of the time and scoring 52.9 percent for White. 1. d4 was next, seen in 36.9 percent of games, but scoring significantly better than the king-pawn opening at 61.2 percent. 1. Nf3 was in distant third, appearing in 7.9 percent of the games and scoring 54.5 percent.

Tournament leaders and selected games:

The second half of stage one, already in progress at www.Chess.com/CCCC, will determine which eight engines advance to stage two. At the halfway mark, it's a familiar group including the big three of Houdini, Komodo, and Stockfish; Leela in fourth place; and four more strong traditional engines in the top eight with Shredder, Fire, Booot, and Ethereal in prime position to advance.

Tournament leader Houdini conjured a nice win against a tough opponent (sixth-ranked Fire) in game 195:

Komodo certainly got 22nd-ranked Senpai to notice it with a brutal checkmate after promoting two pawns into queens:

In game 153, Stockfish squeezed the life out of Crafty, looking more like a snake than a fish in a positional buildup that frightened some viewers in the live chat:

Note to engines: Do not play the Philidor against the world's top computer players!

Shredder, a rare participant in public computer chess tournaments, was an early surprise leader before settling into fifth place. Its win vs Fire showed the power of patience:

Fire was too hot for Fritz to handle after a scorching knight sacrifice on move 10 in the Giuoco Piano:

Booot kicked Texel's king to a punishing back-rank checkmate after securing an early advantage in a sharp Sicilian Dragon opening and then cementing the game with a thematic rook sacrifice:

Ethereal clawed its way into the top eight at the halfway mark by winning seven games against the bottom 10 engines in the crosstable. The engine quickly won against the as-yet-unenlightened Nirvana, which managed just one win and four draws in its first 23 games:

Tournament predictions:

Chess.com ran a prediction contest for fans before the tournament, with members asked to rank the engines 1-24. At the halfway mark, the wisdom of the crowd was spot-on, as Chess.com contest participants correctly predicted seven of the top eight engines, missing only on Ethereal, which was the ninth choice on average.

The average rank of each engine predicted by Chess.com contest participants.

Chess.com predictors were bullish on Lc0, slating Leela as the fourth choice, but even that optimism undersold the engine's success, as it has significantly over-performed its average predicted rank of just below sixth.

CCCC 1 second half:

The second half of CCCC 1: Rapid Rumble is now in progress, running 24 hours a day until we complete game 552. Watch and chat at www.Chess.com/CCCC or our dedicated Twitch channel (complete with AI-generated background music), and stay tuned to Chess.com for more Computer Championship coverage, including highlights and live shows as the tournament heats up.

What do you think of the first Computer Chess Championship event so far? Are you a Leela believer?